The present invention relates to a control rod guide tube designed to be included in the skeleton of a nuclear fuel assembly. It is known that fuel assemblies presently in use include two end pieces interconnected by guide tubes of zirconium-based alloy which generally have grids secured thereto for holding fuel rods in a regular array. At least some of the guide tubes of assemblies placed in a reactor are designed to receive the rods of control clusters as they are lowered. Conventionally, the lower portions of such guide tubes have a narrow zone for the purpose of braking a falling cluster at the end of the displacement thereof by throttling the flow of coolant expelled by the rod, between the rod and the narrow zone.
In the event of clusters falling, this lower portion is subjected to high levels of stress because of the excess hydraulic pressure that then arises. It is also stressed during transient phenomena caused by successively raising the assembly and then allowing it to drop back. While dropping back, the inertia of the assembly runs the risk of causing the weakened portions of the tubes to buckle.
By way of example, FIG. 1 shows the general structure of a guide tube 10 as commonly used at present, and also the manner in which it is fastened. The guide tube is of constant thickness over substantially all of its length. In a running portion 12 whose length L generally constitutes about 80% of the total length of the tube, the nominal outside diameter of the tube is D. In the example shown, the running portion 12 is extended upwards by an enlarged top end zone 14 for attachment to the top end piece 16 of the assembly by means of a screwed socket 25. The major fraction of the length of the lower portion of the guide tube has a smaller diameter d. Because the thickness of the tube is constant, the shrinking of the tube from D to d gives rise to a reduction in the inside diameter thereof. The coolant expelled by the rod 18 moving down along the tube is consequently subjected to considerable head loss once the rod has gone past the running portion 12. The bottom 20 of the tube need not be of reduced size, particularly when the means for fixing it to the bottom end piece 22 are of the kind shown in FIG. 1 where a hollow screw 23 is used to establish a throttled exhaust passage for the coolant, or for attachment to a grid (not shown). The portion 16 may locally present a zone 24 of diameter D, e.g., to enable another grid (not shown) to be fixed thereto.
The means for attaching the guide tube 10 to the end pieces 16 and 22 are of the kind described in FR-A-2 368 785 and 2 465 916, for example, to which reference may be made.
The portion of the guide tube, shown in FIG. 1 whose diameter is reduced to d constitutes a weakened portion that is liable to deform in the event of excessive stress. It might be thought that this risk could be eliminated by locally reducing the inside diameter of the bottom portion of a guide tube having constant diameter D by welding a length of tube inside the lower portion thereof. However, such welding would leave residual stresses that are also harmful, and there would also be geometrical discontinuities of diameter and of straightness.